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Cancelable elements are a new way of proving local updates, by
removing some cancellable element of the global state, provided that
we own it and we are willing to lose this ownership.

Identity-free elements are an auxiliary that is necessary to prove that
[Some x] is cancelable.

For technical reasons, these two notions are not defined exactly like
what one might expect, but also take into account validity. Otherwise,
an exclusive element would not be cancelable or idfree, which is
rather confusing.

Also add "Local" to some Default Proof Using to keep them more contained

This patch was created using

  find -name *.v | xargs -L 1 awk -i inplace '{from = 0} /^From/{ from = 1; ever_from = 1} { if (from == 0 && seen == 0 && ever_from == 1) { print "Set Default Proof Using \"Type*\"."; seen = 1 } }1 '

and some minor manual editing

Use COFEs only for the recursive domain equation solver

These are very useful when dealing with the authoritative CMRA.

This allows us to factor out properties about connectives that
commute with the big operators.

This requirement was useful in Iris 2.0: in order to ensure that ownership of
the physical state was timeless, we required the ghost CMRA to have a timeless
unit. To avoid having additional type class parameters, or having to extend the
algebraic hierarchy, we required the units of any CMRA to be timeless.

In Iris 3.0, this issue no longer applies: ownership of the physical state is
ghost ownership in the global CMRA, whose unit is always timeless.

Thanks to Jeehoon Kang for spotting this unnecessary requirement.

This is more consistent with the definition of the extension order, which
is also defined in terms of an existential.